Edit your skyrim.ini file and add this as a new line under the
[General] section to disable the invisible wall at the edge of Skyrim:
I don’t know whether it’s possible to do this on the Xbox 360. On Windows, I think skyrim.ini is usually in your documents, in the “My Games/Skyrim” folder.
If you can’t edit skyrim.ini on the Xbox 360, you could perhaps try to use the wooden plate glitch to travel through the border region. It works on city walls, but I don’t know whether it works on the border regions.
Prepare to be mildly disappointed: there is nothing much out there. You’ll find a few trees, and then mountains and ocean. The world will lose detail and you’ll eventually reach an endless ocean with nothing in it.
Mods occasionally place things outside those border regions, but in the vanilla game, you won’t find anything much out there.